Thermocouple response time compensation circuit arrangement

ABSTRACT

A circuit for use in a gas turbine engine system with a gas temperature-sensing thermocouple to compensate for time lag in the response of the thermocouple to gas temperature change and, particularly, to compensate for increase in the time lag with decrease in engine speed. In addition to a continuous proportional signal, a periodical pulse signal is produced at a frequency proportional to the engine speed and an amplitude proportional to the change of thermocouple output voltage over each period. The amplitudes of the two signals are sampled by a sample-and-hold circuit to produce an output. Since the frequency of the pulse signal is proportional to engine speed, sampling intervals are longer the slower the engine, with a corresponding increase in the amplitudes of the pulses. A change in thermocouple output voltage is thus accentuated more, the slower the engine speed.

[451 Sept. 12, 1972 THERMOCOUPLE RESPONSE TIME COMPENSATION CIRCUITARRANGEMENT [72] lnventor:

[73] Assignee: Ultra Electronics Limited,

don, England [22] Filed: Aug. 10, 1971 [21] Appl. No.: 170,459

Roy Kendell, Harrow, England Lon- [52] U.S. C1. ..307/295, 60/39.28 T,307/240,

[56] References Cited UNITED STATES PATENTS McLafferty ..60/39.29 Dodd..328/164 X Begley et al ..123/32 EA l-leesh ..307/240 X Kerins ..328/3X 3,648,033 3/1972 Bader ..60/39.28 T

Primary Examiner-Donald D. Forrer Assistant Examiner-R. C. WoodbridgeAttorney-Kemon, Palmer & Estabrook [571 ABsrnAeT A circuit for use in agas turbine engine system with a gas temperature-sensing thermocouple tocompensate for time lag in the response of the thermocouple to gastemperature change and, particularly, to compensate for increase in thetime lag with decrease in engine speed. In addition to a continuousproportional signal, a periodical pulse signal is produced at afrequency proportional to the engine speed and an amplitude proportionalto the change of thermocouple output voltage over each period. Theamplitudes of the two signals are sampled by a sample-and-hold circuitto produce an output. Since the frequency of the pulse signal isproportional to engine speed, sampling intervals are longer the slowerthe engine, with a corresponding increase in the amplitudes of thepulses. A change in thermocouple output voltage is thus accentuatedmore, the slower the engine speed.

7 Claims, 4 Drawing Figures PATENTEU SEP 12 m2 SHEET 1 OF 2 INVEN TORROY KENDELL BY PM,

ATTORNEYS PATENTEU I973 3.691.405

SHEETEUFZ FIG.2.

FIG.3. 54

FIG-4.

INVENTOR ROY KE NDELL THERMOCOUPLE RESPONSE TIME COMPENSATION CIRCUITARRANGEMENT The invention relates to a thermocouple response timecompensation circuit arrangement, particularly for use on a gas turbineautomotive engine, the speed of which is monitored and reproduced inanalogue form as the frequency of an electrical signal. A problem withthermocouples is that the output signal from the thermocouple alwayslags the temperature change at the thermocouple junction. in controlsystems this can be very disadvantageous, and it is an object of thepresent invention to provide a circuit arrangement which utilizes theoutput from a thermocouple to reproduce an electrical signal moreanalogous to the temperature variations occurring at the thermocouplejunction.

The principle of the invention is to compensate for changes in thethermocouple time constants which vary as a function of engine speed.Previously the lag in the response characteristic has been compensatedby introducing into the signal processing circuitry a lead circuit whichprovides a predetermined compensation. However, because the thermocoupletime constant varies with engine speed as the compensation remainsfixed, the system response can be overdamped (under compensated) at lowengine speeds, or underdamped (over compensated) at high engine speeds.

The known lead circuits generally consist of a resistor/capacitornetwork and the present invention provides an arrangement in which theeffective value of the capacitor in the lead network is dependent uponengine speed.

By modifying the known circuit by placing a switch in series with thecapacitor in the lead network and by providing a sample and hold circuitat the output, and controlling the switching according to engine speed,the effective value of the capacitor is set by the ratio of the timebetween samples to the sampling time. That is to say if the engine speedis represented by a signal of variable mark-to-space ratio, this ratiowill determine the effective lead and hence compensation provided by thearrangement. The mark/space signals have a substantially constant markpulse width but a variable space so as to provide the desired variablemark-tospace ratio.

According to one aspect of the invention there is provided athermocouple response time compensation circuit arrangement for use in aturbine engine system of a type including a speed sensor and athermocouple, wherein the speed sensor is adapted to produce a periodicelectrical signal of a frequency proportional to the engine speed andwherein the thermocouple is arranged to sense the temperature of gas ina predetermined part of the engine and to produce a voltage output of amagnitude dependent on the gas temperature, said thermocouple responsetime compensation circuit arrangement comprising:

means for sampling the voltage output of the thermocouple to provide afirst signal proportional to the voltage output;

means for defining time intervals each of a duration inverselyproportional to the speed of the engine in response to said speedsensor;

means for detecting the magnitude of the change if any in the magnitudeof the output voltage of the thermocouple from the beginning to the endof each respective time interval and for providing at the end of eachtime interval a limited duration second signal of a magnitudeproportional to the magnitude of such change;

means for summing said first signal and said second signal;

means for sampling the sum of said first signal and said second signalperiodically and for producing a limited duration third signal of amagnitude proportional thereto; and

means for producing a fourth signal of a longer duration than said thirdsignal and a magnitude equal to the magnitude of said third signal;

whereby a given rate of change in the thermocouple output voltageresults in said second signal having a greater amplitude the lower theengine speed to compensate for an increase in the thermocouple timeconstant with a lower engine speed, so that said fourth signalrepresents a closer approximation to the actual instantaneous gastemperature than said first signal when the gas temperature is changing.

According to another aspect of the invention there is provided athermocouple response time compensation circuit arrangement for use in aturbine engine system of a type including a speed sensor and athermocouple, wherein the speed sensor is adapted to produce a periodicelectrical signal of a frequency proportional to the engine speed andwherein the thermocouple is arranged to sense the temperature of gas ina predetermined part of the engine and to produce a voltage 0utput of amagnitude dependent on the gas temperature, said thermocouple responsetime compensation circuit arrangement comprising:

means for producing a first signal of a magnitude proportional to themagnitude of the thermocouple output voltage;

means for producing a periodic pulse-form second signal at a frequencyproportional to engine speed and an amplitude proportional to the changeif any of the thermocouple output voltage over each period of saidsecond signal; and

means for producing an output signal proportional to the sums of themagnitudes of said first signal and said second signal.

According to another aspect of the present invention, there is provideda thermocouple response time compensation circuit arrangement for use ona turbine engine, the speed of which is monitored and reproduced inanalogue form as the frequency of an electrical signal, said circuitarrangement comprising sampling means for sampling the output signalfrom the thermocouple at a rate dependent upon the frequency of saidelectrical signal, holding means for holding the sampled signal betweensamples, the arrangement being such that the sampling rate is increasedproportionally to the turbine engine speed so that the difference insampled signal levels decreases as the engine speed increases, so that asmaller current is required to reset the hold means so giving a smallerdifferential effeet.

The sampling means preferably includes a pair of field effecttransistors driven as switches from a common source. The holding meansmay be a capacitance in an output circuit of one of the transistors andthe difference in sampled signals may be determined by the voltageacross a capacitance in an input circuit to the other transistor.

The invention will now be described by way of example, with reference tothe accompanying drawings in which:

FIG. 1 shows a circuit arrangement in accordance with the invention;

FIG. 2 shows graphically the temperature variation related to time; and

FIG. 3 shows the electrical output signal from a thermocouple subjectedto the temperature variations as shown in FIG. 2, and the correctedthermocouple response;

FIG. 4 shows graphically the relationship between engine speed N and thethermocouple time constant T.

The illustrated circuit arrangement comprises an input terminal 1connected via a capacitance 2 to the cathode of a diode 3, the anode ofwhich is connected to the base of a PNP transistor 4. The emitter andcollector of transistor 4 are respectively connected to the emitter andbase of another PNP transistor 5.

A feedback circuit comprises a capacitance 6 connected between theemitter of transistor 4 and an earth potential rail 7 and a resistance 8connected between the rail 7 and the junction between capacitance 2 anddiode 3. The emitter of transistor 4 is connected to a positive supplypotential of +12 volts. The base and collector of transistor areconnected respectively through resistances 9 and 10 to a negative supplypotential of -12 volts. The collector of transistor 5 is also connectedto the cathode of a diode 11, the anode of which is connected through aresistance 12 to the gate of an input field-effect transistor, (FET,")13. One other terminal, (referred to as the source, although FET I3 issymmetrical,) of FET 13 is connected through a resistance 14 andcapacitance 15 in series to an input terminal 16, whilst the thirdterminals, (the drain,) of FET 13 is connected through resistances l7and 18 in series also to the terminal l6. A resistance 19 connects thejunction between diode I1 and resistance 12 to earth at 20.

The junction of resistance l7 and 18 is connected to a non-invertinginput 21 of an amplifier 22. An inverting input 23 of amplifier 22 isconnected through a resistance 24 to earth at 20. Positive and negativevoltage supplies E, 12 volts,) and E 12 volts,) respec tively areconnected to amplifier 22 at 25 and 26.

It is convenient to mention at this point that amplifier 22 is made byFairchild and is their type A 709. The terminals of amplifier 22 shownin the drawings will be recognized by those normally skilled in the art.Feedback is provided between terminals 27 and 28 by a resistance 29 andcapacitance 30 in series. Output terminal 31 of amplifier 22 suppliesfeedback to terminal 32 via a capacitance 33. A resistance 34 isconnected between output terminal 31 and non-inverting input terminal 21ofamplifier 22.

The output terminal 31. of amplifier 311 is also connected to the source35 of another FET 36. The gate 37 of FET 36 is connected through aresistance 38 to output terminal 3! of amplifier 22, gate 37 also beingconnected through a resistance 39 to the anode of a diode 40, thecathode of which is connected to the collector of transistor 5. Thedrain 41 of FET 36 is connected through a capacitance 42 to a steadypotential at 43, conveniently earth, and to the base of an NPNtransistor 44. The collector of transistor 44 is connected to thepositive voltage supply of 12 volts,

whilst the emitter of transistor 44 is connected through a resistance 45to the negative voltage supply of 12 volts. The emitter of transistor 43is also connected through the resistance 45, a capacitance 46 and thecapacitance 42 in that order in series to the base of transistor 44. Theemitter of transistor 44 is also connected to the base of a PNPtransistor 47, the collector of which is connected to the negativevoltage supply of 12 volts. The emitter of transistor 47 is connected toan output terminal 48 and through a resistance 49 to the positivevoltage supply of 12 volts.

The PNP transistors 4, 5 and 44 are each of type BC 2l2. The NPNtransistor 44 is type BC 183. Each of the FETs l3 and 36 is of type 2 N3824. Each of the diodes 3, 11 and 40 is type 1 N 914. The resistanceshave the following values Resistance 8 22 K Resistance 9 47 K Resistance10 470 K Resistance 12 22 K Resistance 14 l K Resistance 17 l KResistance l8 I00 K Resistance 19 270 K Resistance 24 47 K Resistance 291.5 K

Resistance 34 K Resistance 38 270 K Resistance 39 22 K Resistance 45 220K Resistance 49 22 K The capacitances have the following valuesCapacitance 2 6,800 pF Capacitance 6 0.1 ,u. F

Capacitance 15 5 1.1. F

Capacitance 30 0.01 {L F Capacitance 33 330 pF Capacitance 42 1.6 ,u F

Capacitance 46 l p. F

frequency of the pulse signals applied to the input terminal 1 from anengine speed sensor 50, (shown only schematically,) is proportional tothe engine speed N. The time constant determined by the capacitance 2and resistance 8 connected to zero volt supply line 10 controls the ontime T of the transistor 5. The FET 13 couples input terminal 16 to theamplifier 22 so that when the transistor 13 is on, capacitance 15charges to the potential applied to the terminal 16. The signal appliedto the terminal 16 is derived from a thermocouple 51, (also shown onlyschematically) mounted on a turbine engine 52 shown schematically. Whenthe transistor 13 is turned on, the transistor 36 is turned on and theoutput from the amplifier 22 is applied to a hold circuit formed bycapacitance 42. The output from the amplifier 22 is therefore applied toa sample and hold circuit, comprising transistor 36 and capacitance 42.

in operation, when the transistor 13 is on, the capacitor 15 charges upto the input level on the terminal 16. Assuming the input signal fromthe thermocouple 51 does not vary, the capacitor 15 will hold thischarge whilst the transistor 13 is off. When the transistor 13 is againturned on, the capacitor 15 will already be charged to the potentialapplied to the input 16, and so no further charge current is taken. Ifthe input potential on the terminal 16 rises or falls during the offperiod of the transistor 13, the capacitor 15 will charge or dischargeto the new voltage, when the transistor 13 is again turned on, so givinga pulse of current proportional to the difference in input voltagebetween consecutive samples. This pulse of current is summed at theinput of the amplifier 22 with the proportional signal obtained viaresistance 18, so increasing the apparent level if it is falling. Thiscan be seen from FIG. 2, which shows a sharp increase in temperature.This sharp rate of increase of temperature results in a thermocoupleoutput signal represented by the line 53 on FIG. 3. The lag in thethermocouple response is due to engine speed dependent time constants.The circuit arrangement shown in FIG. 1 improves the slow response asshown by the line 53 to produce an output signal corresponding to theline 54 on FIG. 3.

FIG. 4 shows how the thermocouple response time constant varies withengine speed N. It will be seen from this 1' graph that the degree ofcompensation required must be dependent upon engine speed.

Referring again to FIG. 1, the FET switch 36 is turned on and off at thesame time as the FET switch 13, so that the capacitor 42 is charged tothe peak level of the output from the amplifier 22. This level is heldwhile the transistor 36 is off, so that the double emitter followerformed by transistors 44 and 47 and resistances 45 and 49 to the outputterminal 48 provides minimum drain current from the capacitor 42 whilethe transistor 36 is off. The pulses applied to the bases of thetransistors 13 and 36 from the transistor 5 have a frequency dependentupon engine speed and, due to the time constant provided by thecapacitor 2 and resistor 8, the signal consists of constant width markpulses and variable width space pulses. The width of the mark pulses issufficient to allow the capacitors 15 and 42 to fully charge during eachpulse. in a particular embodiment the engine speed is monitored from apulse probe 50 associated with the gasifier turbine. This pulse probe 50provides the pulse frequency signal applied to the terminal 1 and it canbe seen that as the frequency of this signal varies, so varying themark-to-space ratio of the signals applied to the transistors 13 and 36,the sampling frequency of the thermocouple signal applied to theterminal 16 is controlled. It will be appreciated that for any givenrate of change of the thermocouple signal applied to the terminal 16,the current required to reset the charge on the capacitor 15 will bedependent upon the sampling frequency. Hence for increasing enginespeed, smaller current is required to recharge the capacitor 15 sogiving a smaller differential effect and effectively varying the degreeof lead provided by the circuit arrangement.

Preferably, the complete circuit arrangement illustrated within thechain-dotted line is produced as a single module including solid stateelectronic devices.

Iclaim:

1. A thermocouple response time compensation circuit arrangement for usein a turbine engine system of a type including a speed sensor and athermocouple, wherein the speed sensor is adapted to produce a periodicelectrical signal of a frequency proportional to the engine speed andwherein the thermocouple is arranged to sense the temperature of gas ina predetermined part of the engine and to produce a voltage output of amagnitude dependent on the gas temperature, said thermocouple responsetime compensation circuit arrangement comprising:

means for sampling the voltage output of the thermocouple to providefirst signal proportional to the voltage output;

means for defining time intervals each of a duration inverselyproportional to the speed of the engine in response to said speedsensor;

means for detecting the magnitude of the change if any in the magnitudeof the output voltage of the thermocouple from the beginning to the endof each respective time interval and for providing at the end of eachtime interval a limited duration second signal of a magnitudeproportional to the magnitude of such change;

means for summing said first signal and said second signal;

means for sampling the sum of said first signal and said second signalperiodically and for producing a limited duration third signal of amagnitude proportional thereto; and

means for producing a fourth signal of a longer duration than said thirdsignal and a magnitude equal to the magnitude of said third signal;

whereby a given rate of change in the thermocouple output voltageresults in said second signal having a greater magnitude the lower theengine speed to compensate for an increase in the thermocouple timeconstant with a lower engine speed, so that,

said fourth signal represents a closer approximation to the actualinstantaneous gas temperature than said first signal when the gastemperature is changing.

2. A thermocouple response time compensation circuit arrangement asrecited in claim 1 wherein:

the voltage output continuous sampling means comprises a firstresistance;

the time interval defining means comprises a first electrical switchingdevice;

the magnitude change detecting means comprises a capacitance, aresistance and a second electrical switching device all connectedtogether in series arrangement;

said first resistance is connected across said series arrangement toform a parallel circuit having two terminals;

one terminal of said parallel circuit is an input terminal forconnection to the thermocouple;

the other terminal of said parallel circuit is connected to the summingmeans; and

said second electrical switching device is controllably connected tosaid first electrical switching device.

3. A thermocouple response time compensation circuit arrangement asrecited in claim 2 wherein:

said first electrical switching device comprises a pair of transistorsconnected together;

said second electrical switching device comprises a field effecttransistor and a diode, the field effect transistor having a gateelectrode; and

the gate electrode is connected through the diode to said firstelectrical switching device.

4. A thermocouple response time compensation circuit arrangement asrecited in claim 1 wherein the time interval defining means comprises:

an input terminal for connection to the speed sensor;

a transistor switching circuit; and

a time delay circuit;

said time delay circuit comprising a resistance, a

capacitance and a diode;

said time delay circuit connecting said transistor switching circuit tosaid input terminal and being adapted to switch said transistorswitching circuit from a first condition to a second conditionperiodically in response to the speed sensor, to hold said transistorswitching circuit in said second condition for a fixed time intervaleach time that it is switched thereto and then to switch said transistorswitching circuit back to said first conditron.

5. A thermocouple response time compensation circuit arrangement asrecited in claim 1 wherein the periodical sum sampling means comprises:

a field effect transistor having a source, a gate and a drain;

the source being connected to the summing means;

the gate being connected to the time interval defining means; and

the drain being connected to the fourth signal producing means.

6. A thermocouple response time compensation circuit arrangement asrecited in claim 5 wherein the fourth signal producing means comprises:

a capacitance connected to the drain of the field effect transistor; and

a proportional amplifier having an input connected to said capacitanceand having an output for said fourth signal.

'7. A thermocouple response time compensation circuit arrangement foruse in a turbine engine system of a type including a speed sensor and athermocouple, wherein the speed sensor is adapted to produce a periodicelectrical signal of a frequency proportional to the engine speed andwherein the thermocouple is arrange to sense the temperature of gas in apredetermined part of the engine and to produce a voltage output of amagnitude dependent on the gas temperature, said thermocouple responsetime compensation circuit arrangement comprising:

means for producing a first signal of a magnitude proportional to themagnitude of the thermocouple output voltage; means for producing aperiodic pulse-form second signal at a frequency proportional to enginespeed and an amplitude proportional to the change if any of thethermocouple output voltage over each period of said second signal; andmeans for producing an output signal proportional to the sums of themagnitude of said first signal and said second signal.

1. A thermocouple response time compensation circuit arrangement for usein a turbine engine system of a type including a speed sensor and athermocouple, wherein the speed sensor is adapted to produce a periodicelectrical signal of a frequency proportional to the engine speed andwherein the thermocouple is arranged to sense the temperature of gas ina predetermined part of the engine and to produce a voltage output of amagnitude dependent on the gas temperature, said thermocouple responsetime compensation circuit arrangement comprising: means for sampling thevoltage output of the thermocouple to provide first signal proportionalto the voltage output; means for defining time intervals each of aduration inversely proportional to the speed of the engine in responseto said speed sensor; means for detecting the magnitude of the change ifany in the magnitude of the output voltage of the thermocouple from thebeginning to the end of each respective time interval and for providingat the end of each time interval a limited duration second signal of amagnitude proportional to the magnitude of such change; means forsumming said first signal and said second signal; means for sampling thesum of said first signal and said second signal periodically and forproducing a limited duration third signal of a magnitude proportionalthereto; and means for producing a fourth signal of a longer durationthan said third signal and a magnitude equal to the magnitude of saidthird signal; whereby a given rate of change in the thermocouple outputvoltage results in said second signal having a greater magnitude thelower the engine speed to compensate for an increase in the thermocoupletime constant with a lower engine speed, so that said fourth signalrepresents a closer approximation to the actual instantaneous gastEmperature than said first signal when the gas temperature is changing.2. A thermocouple response time compensation circuit arrangement asrecited in claim 1 wherein: the voltage output continuous sampling meanscomprises a first resistance; the time interval defining means comprisesa first electrical switching device; the magnitude change detectingmeans comprises a capacitance, a resistance and a second electricalswitching device all connected together in series arrangement; saidfirst resistance is connected across said series arrangement to form aparallel circuit having two terminals; one terminal of said parallelcircuit is an input terminal for connection to the thermocouple; theother terminal of said parallel circuit is connected to the summingmeans; and said second electrical switching device is controllablyconnected to said first electrical switching device.
 3. A thermocoupleresponse time compensation circuit arrangement as recited in claim 2wherein: said first electrical switching device comprises a pair oftransistors connected together; said second electrical switching devicecomprises a field effect transistor and a diode, the field effecttransistor having a gate electrode; and the gate electrode is connectedthrough the diode to said first electrical switching device.
 4. Athermocouple response time compensation circuit arrangement as recitedin claim 1 wherein the time interval defining means comprises: an inputterminal for connection to the speed sensor; a transistor switchingcircuit; and a time delay circuit; said time delay circuit comprising aresistance, a capacitance and a diode; said time delay circuitconnecting said transistor switching circuit to said input terminal andbeing adapted to switch said transistor switching circuit from a firstcondition to a second condition periodically in response to the speedsensor, to hold said transistor switching circuit in said secondcondition for a fixed time interval each time that it is switchedthereto and then to switch said transistor switching circuit back tosaid first condition.
 5. A thermocouple response time compensationcircuit arrangement as recited in claim 1 wherein the periodical sumsampling means comprises: a field effect transistor having a source, agate and a drain; the source being connected to the summing means; thegate being connected to the time interval defining means; and the drainbeing connected to the fourth signal producing means.
 6. A thermocoupleresponse time compensation circuit arrangement as recited in claim 5wherein the fourth signal producing means comprises: a capacitanceconnected to the drain of the field effect transistor; and aproportional amplifier having an input connected to said capacitance andhaving an output for said fourth signal.
 7. A thermocouple response timecompensation circuit arrangement for use in a turbine engine system of atype including a speed sensor and a thermocouple, wherein the speedsensor is adapted to produce a periodic electrical signal of a frequencyproportional to the engine speed and wherein the thermocouple isarranged to sense the temperature of gas in a predetermined part of theengine and to produce a voltage output of a magnitude dependent on thegas temperature, said thermocouple response time compensation circuitarrangement comprising: means for producing a first signal of amagnitude proportional to the magnitude of the thermocouple outputvoltage; means for producing a periodic pulse-form second signal at afrequency proportional to engine speed and an amplitude proportional tothe change if any of the thermocouple output voltage over each period ofsaid second signal; and means for producing an output signalproportional to the sums of the magnitude of said first signal and saidsecond signal.